Coating ferrous metals with aluminum



CGATENG FERRGUS ME'EAZLS WITH ALUMINUM Harald Lundin, .lersey City, N. 3.; Helen Marie Lundin, executrix of the estate of'said Harald Lundin, deceased, assignor of one-half to Helen Marie-Lundin, WestNew York, N. J., and oneahalf. to' Birgit Waller, Uppsala, Sweden No Drawing. Application December 13, 1952, Serial No. 325,885

8 Claims. (Cl. 117-51) This invention relates to coating-iron and iron alloys and related metals and alloys with aluminum. In accordance with the invention, a continuous, tightly adherent coating of aluminum isformed on such metals by applying a flux comprising. a double'fluoride of an alkali metal and aluminum to a surface of the metal, and then bringing the flux-coated base metal into contact with molten aluminum.

Much eflort has been devoted heretofore to the development of methods for forming rust-resistant coatings of aluminum on ferrous metal articles, with varying degrees of success. Many such efforts have been concerned with methods which involve applying a flux to the surface of the ferrous metal, and then immersing the flux-coated metal in molten aluminum. Most proposals of methods of this character have been found to be impractical for commercial exploitation, generally because the flux employed. has proved in practice to contribute little if anything to promoting a truly continuous and adherent coating of aluminum on the ferrous metal base. An outstanding exception to this generality is the method described in' my co-pending application Serial No. 267,303, filed January 19, 1952, now Patent No. 2,686,355, which involves the use of aflux selected from the group consistingof fluoride compounds of zirconium and fluoride compounds of titanium. So far as'I am aware, the flux compositions of my. said co-pending application are the only flux compositions proposed heretofore which promote the formation on ferrous metal articles of aluminum coatings which are superior in continuity, adherence to the base metal, and ease with which they are formed, to aluminum coatings applied directly to a. carefully cleaned ferrous metal article without'the assistance of any flux whatever.

I have now discovered that aluminum coatings of excellent quality may be formed on ferrous'metal articles by first applying to the surface of such. articles a flux comprising a double fluoride of an alkali metal and aluminum containing from 35% to 70%. by weight of A116 and then bringing the flux-coated shape into contact with molten aluminum. Based on this discovery, the method of this invention provides'for" forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of: cobalt, chromium, titanium, nickel, iron, and. alloys thereof, which comprises thoroughly cleaning the. surface of said base metal, then applyingto such cleaned surface a. flux comprising a double fluoride of. an alkali metal and aluminum containingfrom 35% to 70% by weight of Al-Fs and having themelting point below 1400 'F., and then contacting such surface. of thebase metal with molten aluminum. The double fluorideis mostadvantageously applied to the cleaned surface of the base metal by thoroughly Wetting such surface with an aqueous suspension of the fluoride and then evaporating the water from the wetted surface. In thisma'nner a deposit of the double fluoride flux isformed on the surface of the metal.

States Patent Patented Mar. 12, 1957 By the terms double fluoride and alkali metalaIuminum fluoride I mean not only distinct chemical compounds of the alkali metal, aluminum, and fluorine, but also simple physical mixtures of alkali metal fluoride and aluminum fluoride; For purposes of thepresent invention, it is immaterial whether thealuminum fluoride and the alkali metal fluoride be chemically combined into an identifiable compound, or be simply physically mixed together-the fluxing action'is the same in either case.

It is sometimes :desirableto employ, in admixture with the double fluoride, a halide of a metal more electropositive than aluminum. Such halides are effective for depressing the melting point of the mixture, and thus generally they promote the formation of satisfactory coatings at somewhat lower temperatures than can be used successfully when the halide is omitted from the flux. A particularly convenientway for incorporating the desired halide inthe'fluxmixture isto suspend the double fluoride inan aqueous solution of the halide, and after wetting with such solution the surface of the metal to be coated, to evaporate water from: the wetted surface, whereby a thin deposit of a flux mixture'of the double fluoride and thehalide is formed on suchsurface.

In carrying out the'method ofthe invention, the metal article to be coated is first: thoroughly cleaned. Conventional degreasing and: cleaning methods can be used. In general, cleaning by vapor. phase or liquid phase degreasing in organic solvents, followedif necessary by an acid pickling operation, results in. satisfactory cleaning ofthe metal preparatory to the aluminum coating operation. In the. case of. steel and the common iron alloys, a particularly satisfactory cleaning: schedule involves first heating the metal in an oxidizing flame at a high enough temperature and for a long enough period of time to burn ofl all organicmat'ter, then pickling the metal in dilute hydrochloric acid for asufiicient period of time to dissolve surface oxides, and then rinsing the pickled article one or more-times in water. to wash ofl'i residual. acid. So far as the present invention is conis not a matter of importance. Any cleaning procedure thatv leaves the surface of the metal free-from substances which interfere with application of the flux and with effective adherence of the aluminum coating is satisfactory and can be employed with success.

After the metal article to be coated has been cleaned, the next step is to apply the flux composition to its surface. As stated heretofore, the flux used in accordance with the present invention comprises a double fluoride of an alkali metaland aluminum containing from 35 to 70% by weight of AlFa, the melting point of the flux being below 1400 F. Of the various alkali metalaluminum double fluorides, the potassium aluminum fluorides have proved to be generally the most satisfactory. Such fluorides can be regarded as being essentially mixtures of potassium. fluoride and aluminum trifluoride in any proportions; Perhaps the best known of the potassium aluminum fluorides is that in which the molecular proportion of potassium. fluoride to aluminum fluoride is 3 to 1, correspondingto the formula KsAlFe. This particular potassium aluminum fluoride, however,,is not suited for use. in accordance. with the invention, for it contains only about 32.5% by weight of A1353 and has a melting, point of approximately 1000 (about 1800 F). The most satisfactory potassium aluminum fluorides for use in accordance with the present invention are those containing from 45% to 60% by weight of AlFs, for the melting points of these compositions for the most part fall below 1300' F., and some of them are molten attemperatures considerably below flie melting pointof aluminum (about IZZ'OFL).

Except for eutectic compositions, the various potassium aluminum fluorides, and indeed the alkali metal aluminum double fluorides generally, do not possess sharp melting points, but become molten inthe course'of being heated through a melting temperature range. Throughoutthis specification and in the claims, the terms melting point and melting temperature are used to denote the temperature at which the double fluoride (or the flux composition in which it is embodied, as the case may he) becomes completely melted. It is therefore-used to denote the upper limit of the melting range over which the composition passes from the non-fluid solid state to the completely fluid liquid state.

I have found that it is necessary for the flux composition employed in carrying out the method of this inven tion tobe molten at the temperature of the molten aluminum in which the article to be coated is immersed. If the melting temperature of the flux composition exceeds the temperature of the molten aluminum, then upon immersion of the flux-coated article into the molten aluminum, the latter remains in its solid state and is incapable of serving as a flux to promote the formation of a continuous, adherent coating of aluminum on the base metal. Indeed, for satisfactory coating operations, the melting temperature of the flux composition should be substantially below (at least 25 F. below) the temperature to which the molten aluminum is heated. Aluminum melts at about 1220 F., and it is generally undesirable in commercial operations to heat it to more than about 200 F. above its melting point for purposes of applying it as a coating to steel or other metal articles. Consequently, the melting temperature of the flux composition employed should not exceed 1400 F.

Referring again to the potassium aluminum fluorides which have been found to be especially suitable for use in flux compositions according to the present invention, a particular advantage of these compounds is that when they contain from 45% to 60% by weight of AlFs, they mostly possess melting temperatures below 1300 F., and when the content of AlFs falls between 50% and 60% by wei ht of the double salt, the melting temperature is below 1250" F. Double fluorides of potassium and aluminum containing between 52% and 58% AlF3 possess melting temperatures below 1200 F., and a minimum melting temperature near 1000 F. is found for potassium aluminum fluoride containing about 55% by weight AlFs. Potassium aluminum fluorides of these low melting compositions are, for the reasons stated, particularly satisfactory compounds to employ in carrying out the method of this invention.

Sodium aluminum fluorides also may be used with success for carrying out the method of the invention. None of the sodium aluminum fluorides have melting temperatures as low as those of the potassium compounds containing between 50% and 60% by weight of AlFs. However, sodium aluminum fluorides containing between 55% and 70% by weight of AlFs have melting temperatures sufliciently below 1400 F. to be useful in accordance with this invention.

Lithium aluminum fluoride shows a minimum melting temperature at two rather widely different compositions, i. e., at about 35% by weight of AlFs and again at about 65% by weight of MP3. At and near both of these compositions the melting point of the compound is well below 1400 R, and both of these compositions can be used with success in carrying out the method of the invention.

It is quite often desirable to employ as the flux a mixture of the alkali metal aluminum fluoride with a halide of a metal more electropositive than aluminum. The proportions in which these two ingredients are mixed to form the flux is not particularly critical; but when it is used, such halide generally constitutes from to 75% i by weight of the mixture. The ir'nportance ofthe halide isthat it produces a mixture having a melting point lower than that of the double fluoride alone, and thereby enhances the fluxing action of the latter. The particular proportions in which the ingredients are mixed should therefore be chosen so that the mixture melts at a lower temperature than does the double fluoride by itself.

The halide used in preparing the mixture should be a salt of a metal more electropositive than aluminum in order to avoid reduction of the halide by the molten aluminum during the course of the aluminum coating opera tion. Particularly satisfactory halides to employ are those of the alkali metals, and especially potassium chloride.

' Sodium chloride also may be used with advantage. Alkasurface ofthe metal.

line earth metal halides in general are less desirable than the corresponding alkali metal compounds; but alkaline earth metal compounds such for example as calcium fluoride have been used with success. Double fluorides of an alkali metal and beryllium have proved to be very satisfactory, such for example as sodium or potassium fluoberyllate.

The flux may be applied in any desired manner to the surface of the metal article which is to be aluminum coated. 7 It may for example be applied by simply sprinkling or dusting the dry flux composition in finely granular or powdered form on to the surface of the metal to be coated. This procedure has the disadvantage of being rather wasteful of the flux. A preferable method is to suspend the double fluoride in an aqueous medium, agitated to prevent settling, then to thoroughly wet the surface of the article with such suspension, and then to evaporate the water from the wetted surface. Thereby a' thin deposit of the double fluoride is formed on the When the flux composition is to comprise a mixture of the double fluoride with an alkali metal halide, the aqueous medium employed is with advantage an aqueous solution of the latter salt. When such a suspension is applied to a metallic surface and the water is evaporated therefrom, a deposit comprising a mixture of the double fluoride with the alkali metal salt is formed thereon. For example, if potassium aluminum fluoride is suspended in an aqueous solution of potassium chloride and the surface of a steel article is wetted with such suspension, then after the water has been evaporated from the wetted surface a thin deposit of a mixture of potassium chloride and potassium aluminum fluoride will be left thereon. 7

While aqueous suspensions of the flux salt are generally the most economical, it is equally feasible to employ suspensions in non-aqueous volatile liquids, such for example as methyl, ethyl, or other low boiling alcohol, a liquid hydrocarbon such as hexane, carbon tetrachloride, etc. The use of methanol as the liquid in which to suspend the flux composition is of particular advantage, for example, when it is prepared from anhydrous aluminum fluoride and potassium fluoride. Of course, the fire hazard must be taken into account in using methanol, a hydrocarbon, or other inflammable liquid to suspend flux com position.

The flux suspension in either aqueous or non-aqueous liquid medium may be prepared by intimately mixing an alkali metal fluoride and aluminum fluoride (as by ballmilling them together) and suspending the resulting mixturein. the selected medium; or by similarly intimately mixing KsAlFs with an excess of aluminum fluoride and suspending such mixture in the chosen medium. Again, aluminum fluoride which is substantially insoluble in water, may be suspended in a solution of an alkali metal fluoride. Which of these methods for preparing the flux suspension is used is immaterial so far as the present invention is concerned. The choice between them, and other methods of-preparing the suspension which will occur to those skilled in the art, will be determined by considerations of convenience. t

The amount of flux employed. in formingaluminum coatings in accordance with the invention is not particu larly critical. Generally speaking, the amount of flux used should be at least 0.05 gram per square foot. However, in some cases satisfactory aluminum coatings may be formed when an even smaller .amount of flux is employed. It is possible to form hot-dipped aluminum coatings on steel articles without the use of any flux Whatever (though it is difiicult in this manner to form coatings which provide effective protection against corrosion of the underlying metal), and hence any amount of the flux composition described above is helpful for improving the ease with which the aluminum coating is formed and the quality of corrosion protection which it affords. There is, of course, technically no upper limit on the amount of flux that may be employed, but for reasons of economy a thin uniform application of fillX, not exceeding in amount about one gram per square foot of metal to be coated, should be used.

After the flux has been applied to the metal article, the coating operation is completed by bringing such article into contact with molten aluminum. This, of course, is most easily accomplished by dipping the flux-coated article into a bath of molten aluminum.

Particularly good coatings are obtained when the base metal coated with a film of the flux is agitated in the molten aluminum. This agitation first promotes effective contact between the aluminum, the flux, and the base metal, and finally promotes separation of the flux from the surface of the base metal. Identical results are obtained when the .molten aluminum in the immediate vicinity of the base metal is agitated. Such agitation'of the aluminum is easily obtained when the aluminum is melted in electric induction furnaces.

The coating operation can be carried out as a batch operation or continuously, depending on the nature of the article and the quantity of articles or the amount of metal that is to be coated. Batch coating operations generally are most satisfactory when comparatively few articles, or articles which do not lend themselves to handling on continuous materials-handling apparatus, are to be coated. Continuous coating operations are however most economical when the amount of metal to be coated is large.

Whether the coating operation is being carried out in a batch or continuous operation, the time of immersion of the article to be coated should not be long. An immersion time in the range from 5 to seconds generally is optimum, though for very massive articles a somewhat longer time of immersion, and for very light articles a somewhat shorter time of immersion, may be preferred.

The thickness of the coating is to a large extent determined by the rate at which the base metal is withdrawn from the bath of molten aluminum. Rapid withdrawal favors the formation of a thick coating, and slow withdrawal favors the formation of a thin coating. In coating wires, strip, and the like which are passed continuously through a bath of molten aluminum, therefore, the thickness of the aluminum coating may be controlled by controlling the speed of passage of such article through the bath and thus its rate of withdrawal from the bath.

When wire or strip metal is passed at high speed, say upwards of 50 feet per minute, through the aluminum bath, a lateral oscillation or whip tends to develop. if the vibration or oscillation frequency of such whip is low, there is a tendency for the coating formed on the wire or strip to be rough, whereas if it is a high frequency oscillation, the formation of a smooth coating is favored. It is accordingly advantageous to maintain the wire or strip under as great tension as can safely be applied at the coating temperature, in order to insure that the period of vibration of any whip that occurs will be as high as possible.

A continuous film of molten aluminum adheres tenaciously to the metallic article as it is withdrawn from the bath of molten aluminum. This coating klm is cooled in any desired manner to below the melting point of aluminum upon removal of the article from the bath. Such cooling ordinarily occurs rapidly enough in the open air. If desired however, the article may be cooled by an air blast, or by a water spray, or by other special means, upon its withdrawal from the molten aluminum bath.

Aluminum coatings formed on steel or other base metal by the method of this invention are tightly adherent to the base metal, so that the latter may be severely bent or otherwise deformed without injuring the coating or impairing its effectiveness for protecting the base metal from corrosion. As a matter of fact, when wire, sheet or strip is aluminum-coated by the method of the invention, it is advantageous to subject the coated metal to one or more wire-drawing passes or one or more rolling mill passes at room temperature after the coating has been applied, for in this manner the surface smoothness and luster of the coating are markedly enhanced without impairing its protective qualities. Also, since the temperature of the molten aluminum is above the temperature at which coldworked steel becomes annealed, it is desirable to subject ferrous metal articles that have been aluminum-coated by the method of this invention to plastic deformation at room temperature, in order to increase the tensile strength of the base metal.

Coatings properly made in accordance with the method of this invention are truly continuous, being free even of minute pinholes which can form focal points for corrosion to begin. Aluminum-coated steel wires, strips, or sheets made in accordance with the invention, especially if they have been subjected to plastic deformation by a wire drawing or rolling operation subsequent to application of the aluminum coating, have the attractive brightness and luster of cold-finished metallic aluminum. They are at the same time substantially as resistant to corrosion as is solid metallic aluminum, and yet possess the mechanical strength and economy of the steel base metal.

I claim:

1. The method of forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of cobalt, chromium, titanium, nickel, iron, and alloys thereof, which comprises thoroughly cleaning the surface of said base metal, then applying to such cleaned surface a flux comprising primarily a double fluoride of potassium and aluminum containing from 45% to 60% by weight of MP3 and having a melting point below 1300 F., and then contacting such surface of the base metal with molten aluminum.

2. The method of forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of cobalt, chromium, titanium, nickel, iron, and alloys thereof, which comprises thoroughly cleaning the surface of said base metal, then thoroughly wetting such cleaned surface with an aqueous suspension of a flux comprising primarily potassium aluminum fluoride containing 45% to 60% by weight of AlFs and having a melting point below 1300 R, then evaporating the water from the thus-Wetted surface, whereby a thin deposit of said fluoride is formed on said surface, and then immersing such surface in molten aluminum.

3. The method of forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of cobalt, chromium, titanium, nickel, iron, and alloys thereof, which comprises thoroughly cleaning the surface of said base metal, then applying to such cleaned surface a flux comprising a mixture of a major proportion of potassium aluminum fluoride containing 45 to 60% by weight of AlF3 and a minor proportion of halide of a metal more electropositive than aluminum, said flux having a melting point below 1250 F, and then contacting such surface of the base metal with molten aluminum.

4. The method of forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of cobalt, chromium, titanium,

,7 V nickel, iron, and alloys thereof, which comprises thoroughly cleaning the surface of said base metal, then applying to such cleaned surface a flux comprising a mixture of a major proportion of potassium aluminum fluoride containing 45 to 60% by weight of AIR; and a minor proportion of an alkali metal halide, said flux having a melting point below 1300 F., and then contacting such surface of the base metal with molten aluminum.

5. The method of forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of cobalt, chromium, titanium, nickel, iron, and alloys thereof, which comprises thoroughly cleaning the surface of said base metal, then thoroughly wetting such cleaned surface with a suspension of a major proportion of potassium aluminum fluoride containing from 45% to 60% by weight of AIF: in an aqueous solution of a minor proportion of a halide of a metal more electropositive than aluminum, then evaporating water from the thus-wetted surface, whereby a thin deposit of a flux comprising a mixture of said fluoride and said halide and having a melting point below 1250 F. is formed on said surface, and then immersing said surface in molten aluminum.

6. The method of forming a continuous tightly adherent coating of aluminum on a base metal selected from the group consisting of cobalt, chromium, titanium, nickel, iron and alloys thereof, which comprises thoroughly cleaning the surface of said base metal, thenthoroughly wet ting such cleaned surface with a suspension of a major proportion of potassium aluminum fluoride containing from 45 to 60% by weight of AlFs in an aqueous solution of a minor proportion'of potassium chloride, then evaporating water from the thus-wetted surface, whereby a thin deposit of a flux comprising a mixture of said fluoride and said halide and having a melting point below 1250" F. is formed on said surface, and thentime mersing said surface in molten aluminum.

7. The method of coating a ferrous metal shape with aluminum, which comprises applying to the surface of such shape a flux comprising primarily potassium aluminum fluoride containing 45% to 60% by weight of AlFs, said flux having a melting point below 1300 F., then immersing the shape with flux thereon in molten aluminum, then withdrawing the shape from the molten aluminum with a continuous film of aluminum adhering thereto, and then cooling the resulting aluminum-coated shape to below the melting point of aluminum.

8. The method of coating a ferrous metal shape with aluminum, which comprises applying to the surface of such shape a flux comprising a mixture of a major proportion of potassium aluminum fluoride containing 45% to 60% AlF3 and a minor proportion of potassium chloride, said flux having a melting point below 1250 F., then immersing the shape with flux thereon in molten aluminum, then withdrawing the shape from the molten alu: minum with a continuous film of aluminum adhering thereto, and then cooling the resulting aluminum-coated shape to below the melting point of aluminum.

References Cited in the'file of this patent UNITED STATES PATENTS 503,070 Broadwell Aug. 8, 1893 527,478 Broadwell Oct. 16, 1894 2,569,097 I Grange Sept. 25, 1951 2,686,355 Lundin Aug. 17, 1954 OTHER REFERENCES Einerl et al.: The Chemical Age, April 4, 1942, pp. 

1. THE METHOD OF FORMINF A CONTINUOUS TIGHTLY ADHERENT COATING OF ALUMINUM ON A BASED METAL SELECTED FROM THE GROUP CONSISTING OF COBALT, CHROMIUM, TITANIUM, NICKEL, ION, AND ALLOYS THEREOF, WHICH COMPRISES THOROUGHLY CLEANING THE SURFACE OF SAID BASE METAL,THEN APPLYING TO SUCH CLEANED SURFACE A FLUX COMPRISING PRIMATILY A DOUBLE FLUORIDE OF POTASSIUM AND ALUMINUM CONTAINING FROM 45% TO 60% BY WEIGHT OF ALF3 AND HAVING A MELTING POINT BELOW 1300*F., AND THEN CONTACTING SUCH SURFACE OF THE BASE METAL WITH MOLTEN ALUMINUM. 